The present invention relates to a waveguide type optical device which has an optical waveguide and electrodes formed in a substrate of a ferroelectric crystal which has a pyroelectric effect.
Waveguide type optical devices utilizing an electrooptic effect are a phase modulator, an intensity modulator, an optical switch and so forth. In the case of using, for an optical device, a crystal as of lithium niobate (LiNbO.sub.3) which has a pyroelectric effect, i.e. an effect that spontaneous polarization varies with temperature change, however, the operation of the optical device becomes unstable due to unnecessary electric fields resulting from the generation of electric charges in the crystal surfaces which cross the direction of spontaneous polarization.
FIG. 1 is a diagrammatic showing of a conventional waveguide type optical device 10 having its substrate 11 formed of lithium niobate crystal, for explaining its unstable operation due to a temperature change. The crystal substrate 11 has X-surfaces X1, X2, Y-surfaces Y1, Y2 and Z-surfaces Z1, Z2 perpendicular to the X, Y and Z axes, respectively, and the direction of spontaneous polarization P of the crystal is assumed to be the Z-axis direction. The X-surfaces X1 and X2 in the drawings will be referred to also as the top and bottom surfaces, respectively. An optical waveguide 12 and modulation electrodes 13 and 14 are formed in a surface parallel to the direction P of spontaneous polarization, i.e. in the X-surface X1 in this example. Light which propagates in the optical waveguide 12 is phase modulated by an electrooptic effect (i.e. by the Pockels effect) in accordance with a voltage which is applied to the electrodes 13 and 14.
In a steady state in which the optical device 10 is held at a fixed temperature, polarization charges in the Z-surfaces Z1 and Z2 are neutralized by stray charges in the air which stick to the polarization charges. A temperature change of the crystal substrate 11 causes a change in the amount of polarization, and as a result, positive and negative surface charges develop in the Z-surfaces Z1 and Z2 (which generally, are surfaces crossing the direction of polarization P at an arbitrary angle and which are called polarization planes), respectively, generating electric fields as indicated by their electric lines of force Ef. The resultant electric fields are applied to the optical waveguide 12 directly or indirectly through the electrodes 13 and 14. This changes the phase of the light propagating through the waveguide 12, as is the case with the modulation drive voltage which is applied to the modulation electrodes 13 and 14, and hence is a cause of the unstable operation of the optical device accompanying the temperature change.
With such a structure as shown in FIG. 2, in particular, in which the ferroelectric crystal substrate 11 in FIG. 1 is mounted on a mount 30 having a substantially equal coefficient of thermal expansion, and terminal electrodes 33 and 34 are provided on the marginal portions of the mount 30 along the planes of polarization (i.e. the Y-surfaces Y1 and Y2) and connected by bonding wires 31 and 32 to the modulation electrodes 13 and 14, respectively, so that the terminal electrodes 33 and 34 are each supplied with the modulation drive voltage, the electric lines of force Ef of electric charges resulting from polarization are readily caught by the bonding wires 31 and 32, and consequently, the modulation electrodes 13 and 14 are supplied with a voltage produced by the pyroelectric effect.
In Japanese Application Laid Open No. 73207/87 entitled "Waveguide Type Optical Device" an arrangement is proposed for preventing deterioration of the temperature characteristic of the optical device caused by its pyroelectric effect. According to this prior art literature, a slightly conductive film is formed between the electrodes to that prevent that electric charges generated by the pyroelectric effect from remaining in the electrode portions. With such a structure, however, there is a possibility that if the resistance value of the film is too low, the device will be destroyed by a large current which flows between the electrodes when the electric fields are applied. On the other hand, when the resistance value of the film is too high, the charges caused by the pyroelectric effect cannot completely be driven out of the electrode portions, and hence the intended object cannot be attained. Moreover, if the insulation between the electrodes is lowered by the film, then no effective electric fields are applied to the optical waveguide, resulting in the reduction of the modulation efficiency. Besides, variations in the resistance value of the film lead to variations in the modulation characteristic.